Monday, June 3, 2013

A Dam Big Dam and a Dam Big Bridge...And a Dam Frightening Problem

Oh, how mighty are the works of man...and how arrogant we sometimes get when contemplating our great works. Our first stop on our tour of the western Colorado Plateau was a visit  to Hoover Dam and Lake Mead, the gigantic tower of concrete that blocks the Colorado River, and holds back 30 million acre-feet of water. It's the biggest dam reservoir in the United States. How much is 30 million acre feet? More than two years of "average" Colorado River flows for one. The largest dam in our region back home, Don Pedro on the Tuolumne, holds a paltry 2 million acre-feet.

The dam was built during the height of the Great Depression between 1930 and 1935, and just two years after the catastrophic failure of St. Francis Dam in California that killed around 600 people. Lessons learned from the failure of St. Francis led to modifications of Hoover Dam which presumably make it more or less indestructable.

Just the same, it is amazing how many faults are exposed in the canyon walls adjacent to the dam. The rocks forming the abutments are Miocene volcanic tuff, which doesn't dissolve or crumble in water (a major factor in the St. Francis event). But the rock is highly faulted, and it is a great place to explain slickensides, the scratch marks that occur when blocks of rock slide past each other. The rock is literally polished.
The visitor center is guarded by giant copper art-deco angels that made me feel like I was entering the walls of Minas Tirith in Lord of the Rings.
The thing is dam big. It's more than 700 feet high, which was the highest in the world when it was built (now it is 18th). One can drive or walk across the dam thing, but it is no longer the main highway between Kingman and Las Vegas.
In 2010, work was completed on the O'Callaghan-Tillman Bridge just downstream. It towers 880 feet above the river, and more than 200 feet above the level of the dam. Walking across the top allows the incredible perspective of the dam and lake in the top photo of this post.
 Looking down the dam face is disorienting, but it looks like a great slide (although one's pants would surely catch fire from the friction...). The white thing is part of the power generating complex.
The bridge really is a stunning piece of work, although it is a bit disconcerting to feel it vibrate as the big trucks pass by...
Looking down the other face of the dam is disconcerting too. It's been a long time since the dam was full (1983 to be exact). The water level looks to be only a hundred or so feet low, but that represents perhaps half the storage capacity of the reservoir. A decade long drought has brought down the reservoir to perilous levels (13 million acre-feet), and some forecasts suggest it may never fill again. 30 million people in Los Angeles, Las Vegas, Phoenix and in many agricultural areas depend on the supply of water, but it just isn't there.

And that's dam frightening.

Post a Comment